A recent development that improves the capabilities of inertial systems which stabilize aircraft, missiles, torpedoes, etc. is the fiber interferometer gyro. This is a relatively rugged, reliable and long-term accurate device in which applied rotation rates cause changes in the output intensity of the interferometer. Detection of these changes gives a measure of the rotation rate. One of the first disclosures of such a gyro was by Vali and R. W. Shorthill as "Fiber Laser Gyroscopes" at the East Coast Conference of SPIE in Reston, Virginia on March 22 and 23, 1976.
The essential elements of the Vali et al., fiber interferometer gyro are schematically depicted in FIG. 1 as prior art although the Vali et al., gyro applied a pair of beamsplitters and associated detectors and coupling lenses. The output of a continuous wave laser is divided by a beamsplitter into two beams which circulate in opposite directions in a long, multiple turn, single-mode fiber. The beams exit the fiber and are recombined at the beamsplitter. Their interference pattern is translated by a photodetector into an electronic signal representative of the applied rotation rate.
However, in addition to the beams transmitted through the coiled single-mode fiber, beams are reflected from the fiber ends and from the input/output coupling lenses. These reflected beams also are translated at the photodetector and can interfere with the desired signals (those indicative of rotation rate) and cause erroneous indications of the measurements of the rotation rate.
Schemes for elimination of these reflections mostly have followed two methods. The first, shown as prior art in FIG. 2 of the drawings, employs special antireflection coated lenses and beamsplitters. The fiber ends are polished or cleaved at an angle to deflect the reflected beams along a path which is not coincident with the optical axis of the fiber interferometer. The alternative method calls for the immersion of the entire optical component in a bath of an index matching liquid. The index matching liquid eliminates a variety of reflections since the objectionable reflections occur only at interfaces between media of different refractive indices.
Both of these methods have disadvantages. Both are costly and complex to implement. Generally speaking, shaping the ends degrades the optical surfaces and decreases the gyro signal quality. Grinding and polishing fiber ends at an angle while maintaining a good optical quality surface requires involved fabrication procedures which must be done by skilled techniques. Even when highly skilled technicians and sophisticated equipment are employed and the best fabrication methods are followed, the resulting surfaces generally have scratches and pits making the cleaved fiber end approach unsatisfactory. Immersion of the interferometer in an index matching liquid is not a practical method suitable for a gyro's operational environment. In such an environment there are usually large temperature variations and possible severe mechanical vibrations. These effects introduce variations in the refractive index of the liquid and cause intensity fluctuations with consequent gyro output signal errors.
Thus, there is a continuing need in the state-of-the-art for an apparatus and method for reducing the reflection signals from fiber ends and coupling lenses which is consistent with the rugged, reliable and uncomplicated system of the fiber interferometer gyro.